WASHINGTON/LONDON (Reuters) - Saudi Arabia on Sunday called the killing of journalist Jamal Khashoggi at its Istanbul consulate a "huge and grave mistake", but sought to shield its powerful crown prince from the widening crisis, saying Mohammed bin Salman had not been aware.

The comments from Foreign Minister Adel al-Jubeir were some of the most direct yet from Riyadh, which has given multiple and conflicting accounts about Khashoggi's killing on Oct.2, first denying his death before admitting it on Saturday amid an international outcry.

The kingdom's weeks of denial and lack of credible evidence in the face of allegations from Turkish officials that Khashoggi had been killed have shaken global confidence in ties with the world's top oil exporter, Western governments say.

U.S. Treasury Secretary Stephen Mnuchin said on Sunday that Saudi Arabia's admission that the Washington Post columnist had been killed in a fistfight was a "good first step but not enough", though he added it was premature to discuss any sanctions against Riyadh.

Three major European powers - Germany, Britain and France - pressed Saudi Arabia to provide facts to back up its earlier explanation of a fight, while Chancellor Angela Merkel said Germany would not export arms to Saudi Arabia while the current uncertainty over Khashoggi's fate persisted.

"This is a terrible mistake. This is a terrible tragedy. Our condolences go out to them. We feel their pain," he said. "Unfortunately, a huge and grave mistake was made and I assure them that those responsible will be held accountable for this."

He said the Saudis did not know how Khashoggi, a Saudi national and U.S. resident, had been killed or where his body was. He also said that Prince Mohammed, defacto ruler of Saudi Arabia, was not responsible.

"This was an operation where individuals ended up exceeding the authorities and responsibilities they had. They made the mistake when they killed Jamal Khashoggi in the consulate and they tried to cover up for it."

Khashoggi went missing after entering the consulate to obtain documents for his upcoming marriage.

After denying any involvement in the 59-year-old's disappearance for two weeks, Saudi Arabia on Saturday said Khashoggi, a critic of the crown prince, had died during a fight in the building. An hour later, another Saudi official attributed the death to a chokehold.

"Nothing can justify this killing and we condemn it in the strongest possible terms," Germany, Britain and France said in their joint statement on Sunday.

"There remains an urgent need for clarification of exactly what happened ... beyond the hypotheses that have been raised so far in the Saudi investigation, which need to be backed by facts to be considered credible."

Reflecting international scepticism over its account, a senior Saudi government official has laid out a new version that in key respects contradicts previous explanations.

The latest account includes details on how the team of 15 Saudi nationals sent to confront Khashoggi had threatened him with being drugged and kidnapped and killed him in a chokehold when he resisted. A member of the team dressed in Khashoggi's clothes to make it appear as if he had left the consulate.

ERDOGAN TO SPEAK

Turkish officials suspect Khashoggi was killed inside the consulate by the team of Saudi agents and his body cut up. Turkish sources say the authorities have an audio recording purportedly documentingKhashoggi's murder inside the consulate.

In a speech on Sunday, Erdogan appeared to suggest that he was getting ready to release some information about the Turkish investigation, and would do so at his weekly speech to members of his ruling AK Party in parliament.

He has remained largely silent on the case, although Turkey's pro-government newspapers have released information detailing a 15-member team that purportedly arrived in Istanbul to confrontKhashoggi at the consulate.

"I will make my statements about this issue on Tuesday at the party group meeting," Erdogan said.

For Saudi Arabia's allies - particularly in the West - the question will be whether they believe that the prince, who has painted himself as a reformer, has any culpability. King Salman, 82, had handed the day-to-day running of Saudi Arabia to him.

"I am not satisfied until we find the answer. But it was a big first step, it was a good first step. But I want to get to the answer," U.S. President Donald Trump told reporters this weekend, when asked about the Saudi investigation and Riyadh's subsequent firing of Saudi officials over the incident.

In an interview with the Washington Post, Trump said that "obviously there's been deception, and there's been lies".

His Treasury Secretary, Mnuchin, said he would not attend a Saudi investment conference on Tuesday, although he would still visit Riyadh as planned for talks with his counterpart there on joint efforts to counter terrorist financing.

A leading Republican U.S. senator said he believed the crown prince was behind the killing, adding that the Saudis had lost all credibility in their explanations of his death.

"Yes, I think he did it," Senator Bob Corker, chairman of the Senate Foreign Relations Committee, said in an interview with CNN.

SHIFTING NARRATIVE

Saudi Arabia's account of the incident has changed multiple times. The authorities initially dismissed reports that Khashoggi had gone missing inside the consulate as false and said he had left the building soon after entering. When the media reported a few days later that he had been killed there, they called the accusations "baseless".

King Salman ordered the dismissal of five officials, including Saud al-Qahtani, a royal court adviser seen as the right-hand man to Prince Mohammed, and deputy intelligence chief Ahmed Asiri, Saudi state media reported on Saturday.

The king also ordered a restructuring of the intelligence service, to be led by Prince Mohammed, suggesting the prince still retained wide-ranging authority.

Some governments and prominent executives have said they would pull out of a forthcoming investment conference in Saudi Arabia. Among the latest, the government of New Zealand said it would not attend, as did the head of the main banking unit of Japanese financial group MUFG.

According to the senior Saudi official, the Saudi team rolled up Khashoggi's body in a rug, took it out in a consular vehicle and handed it to a "local cooperator" for disposal.

Turkish investigators are likely to find out what happened to the corpse "before long", a senior Turkish official said on Saturday.